This invention relates to aircraft navigation, in general, and more particularly to a calibration system for distance measuring equipment (DME) used in aircraft navigation.
One of the types of navigation equipment most commonly used with aircraft is what is known as Tacan. Tacan is a system which is utilized to give both direction and range to a ground station. Although the directional information is very accurate, until recently, the range information, which is obtained by distance measuring equipment or DME, has not been particularly accurate. There have, however, been attempts to make this more accurate. For example, see U.S. Pat. No. 4,162,495, which describes a digital Tacan system which gives increased accuracy. This patent also gives some background on the nature of the Tacan system. Basically, an aircraft system generates pairs of pulses which are received by a ground transponder station which, after a predetermined delay, transmits a response pair of pulses. By measuring the time between the transmission of the pair of pulses and the reception of a pair of pulses, it is possible to determine the distance from the ground transponder. In addition to the pulses from that particular aircraft, there are also transmitted pulses in response to other aircraft and random pulses. The received signal also contains squitter, i.e., noise.
The time delay between transmission of interrogation pulses and reception of response pulses is a summation of installation delay, range delay, transponder delay, and receiver delay. Transponder delay is known and fixed, i.e., the transponder responds a known time after receiving pulses. Range delay is, of course, the delay to be measured to determine range. There remains receiver and installation delay. It is the calibration of these delays which leads to errors. In present state-of-the-art systems, nominal receiver delay calibration is built in. Installation delays, which are small compared to receiver delays, have been neglected. As a result, installation delays are not corrected at all and receiver delays are only approximately corrected. There is no way of adding additional correction for the errors, which result as a function of receiver center frequency, ground station transponder frequency, signal dynamics and temperature effects.
Thus, it is the object of the present invention to provide a calibration system which calibrates for receiver and installation delay in distance measuring equipment to give increased range accuracy measurements.